(1) Field of the Invention
The present invention relates generally to antennas, and more particularly to a high-frequency transmit antenna system for over the horizon communication.
(2) Description of the Prior Art
The use of high frequencies between 2 MHz and 30 MHz to provide long distance communications is well known in the art. Ranges of thousands of miles are supported using sky-wave propagation and lesser ranges on the order 100 miles are supported using surface wave propagation over sea water. Suitable antennas are well known in the art and include standard types such as simple dipoles and monopoles along with larger types such as the Yagi-Uda antenna and log-periodic antenna which provide improved performance (i.e., greater gain or bandwidth or both). Since these wavelengths are quite large (i.e., 150 meters at 2 MHz and 10 meters at 30 MHz), effective communications using these well-known antennas requires their dimensions to be suitably large and, ideally, resonant (i.e., wavelength for the dipole and ¼ wave for the monopole). These dimensions unfortunately are not compatible with many applications such as small mobile platforms (e.g., “man pack” or hand held devices) and small marine platforms (e.g., unmanned buoys or vehicles).
The use of electrically-small antenna structures to meet size requirements typically uses passive matching networks to “tune” these structures such that they present a good match to standard 50-ohm transmitters at the desired operating frequency. The drawbacks to this traditional approach include poor efficiency, low bandwidth, and a high susceptibility to small environmental changes (e.g., proximity to a changing waterline or adjacent object such as a structure or person). These drawbacks become more problematic as the desired antenna structure is made smaller and the subsequent departure from the standard 50-ohm transmitter impedance increases.
In an effort to address some of these drawbacks, U.S. Pat. No. 7,804,454 entitled “Active High Frequency Transmitter Antenna Assembly” to Paul M. Mileski, describes an antenna system that utilizes an active electronic circuit close to the antenna structure that overcomes the drawbacks of a passive matching network in order to provide a highly unconventional Ultra-Wide-Band (UWB) signal as part of a wide-band pulse modulated communication system. The active transmit electronics improve the efficiency, bandwidth, and flexibility of the electrically-small antenna system. However, the antenna system does not have the ability to transmit conventional relatively narrow band (or non UWB) high duty cycle signals necessary to support common modulation types such as Phase Shift Keying (PSK) and Frequency Shift Keying (FSK).